The term “(meth)acrolein” conventionally means acrolein or methacrolein, of formula CH2═CH—CHO or CH2═CH(CH3)—CHO, respectively.
The major known industrial processes for producing acrolein are based on various reaction schemes using different raw materials.
Processes which implement an aldol condensation reaction, as described for example in patent UK 513 772 from Degussa, use as raw materials acetaldehyde and formol which react in the gas phase by heterogeneous catalysis to form acrolein. Formol and acetaldehyde are neither stored nor transported easily; for example, formol can be transported only over a radius of 400 km around the production site, and for that must be stabilized with methanol in aqueous solution. In order to dispense with the purchasing of the raw materials, it is necessary to provide for specific units for synthesis of these products, which add to the cost of the aldol condensation unit per se. The synthesis of acetaldehyde is generally carried out by the Wacker reaction for oxidation of ethylene and the synthesis of formol is generally carried out by oxidation (or oxydehydrogenation according to the catalytic system retained) of methanol. The process for synthesis of acrolein according to the adol condensation route therefore requires considerable investments.
The industrial processes based on the oxidation of olefins, in particular the oxidation of propylene, resulting in acrolein, which has been the subject of numerous publications or patents, consume fossil raw materials extracted from oil in particular, of which the cost involved in obtaining access is becoming increasingly high. Moreover, their availability is linked to refining or petrochemical facilities with large capacities, which in practice imposes an establishment in the vicinity of large petrochemical sites, and therefore costs for transportation to the final consumer, not to mention the risks of polluting product emissions.
More recently, a new route of synthesis based on the dehydration reaction of glycerol to acrolein has been the subject of considerable works, in particular by the applicant (WO 2006/087083; WO 2006/087084; WO 2009/128555; WO 10/046227; WO 11/083225). This type of process has the advantage of being able to work with a renewable (non-fossil) natural raw material and therefore to comply with the commitments of most industrialized countries aiming to reduce greenhouse gas emissions with the environmental effects thereof, but also to provide a more sustainable production solution.
However, this route of synthesis has certain disadvantages on several levels. Indeed, the supply of glycerol depends mainly on oleochemical and/or biodiesel plants. Glycerol is only available in limited amounts which in practice restricts the possible size of the acrolein production plants and thus their economic profitability. The most widespread grades of glycerol are those known as crude glycerol, which most commonly corresponds to aqueous solutions of glycerol at 80% by weight containing salts (for example NaCl, KCl, Na2SO4, K2SO4, etc.), residual methanol when it has been obtained by methanolysis of vegetable oils, Matter Organic Non-Glycerol (MONG), and all kinds of impurities extracted from plants or animal fats during the production processes. These impurities are catalyst poisons and lead to reversible deactivation, for example by coke formation, or irreversible deactivation, for example by salt deposition. Refined glycerol is also commercially available, but at much higher prices. From a technical point of view, the dehydration catalysts known to date deactivate quickly, which necessitates regular regenerations, meaning an additional investment cost (for example doubling or tripling of the catalytic volumes). Finally, this “green” process has a drawback in that the dehydration reaction, which is carried out in the gas phase and is overall highly energy-consuming, is dependent on the cost of this energy, currently provided by fossil fuels, which also necessarily impact on the environment.
Moreover, alcohols, in particular a mixture of methanol and ethanol, have already been described as raw materials for forming acrolein. Mention may be made, for example, of application WO 2005/040392 which describes the oxidation of a mixture of methanol and ethanol in the presence of a silver catalyst, producing a mixture of acetaldehyde and formaldehyde, which is then converted to acrolein in the presence of alumina. Example 32 of said document uses a reaction mixture of which the composition is not specified, and no indication regarding the yield of the process is mentioned. This process has the drawback that the reaction temperature must be adapted to each of the steps.
With regard to the synthesis of methacrolein, industrial processes generally use, as raw materials, isobutane, tert-butanol or isobutene which are converted to methacrolein by oxidation. Other synthesis processes exist, such as oxydehydrogenation of isobutaraldehyde or hydroformylation of methylacetylene or of propadiene. Besides, in certain cases, their cost, the fossil origin of these raw materials is a major disadvantage.
Methacrolein can also be obtained by means of an aldol condensation reaction, in particular between formaldehyde and propionaldehyde according to the process described in patent U.S. Pat. No. 4,433,174, with however the drawbacks associated with the use of these aldehydes.
There therefore remains a real need to overcome the drawbacks of the (meth)acrolein synthesis processes of the prior art, in particular to reduce their impact on the environment and to limit the investments and the operating costs involved therein.
One of the objectives of the present invention is therefore to provide a process for synthesis of (meth)acrolein, using raw materials of renewable (non-fossil) origin, which is simple, rapid (comprising as few steps as possible) and easy to carry out and which gives the desired product with good selectivity.
The inventors have discovered that it is possible to synthesize acrolein and methacrolein by coupling an oxidation reaction and an aldol condensation reaction, using a reactive mixture comprising at least one compound chosen from ethers, acetals or hemiacetals derived from linear alcohols comprising from 1 to 3 carbon atoms, some of which are compounds available from renewable raw materials.